This invention relates generally to magnetoresistive (MR) read heads, and more particularly to methods and apparatus for providing a SAL bias for a MR sensing element.
Magnetic disk drives are used to store and retrieve data for digital electronic apparatus such as computers. In FIGS. 1A and 1B, a magnetic disk drive D of the prior art includes a sealed enclosure 1, a disk drive motor 2, a magnetic disk 3, supported for rotation by a spindle S1 of motor 2, an actuator 4 and an arm 5 attached to a spindle S2 of actuator 4. A suspension 6 is coupled at one end to the arm 5, and at its other end to a read/write head or transducer 7. The transducer 7 is typically an inductive write element with a sensor read element. As the motor 2 rotates the disk 3, as indicated by the arrow R, an air bearing is formed under the transducer 7 to lift it off the surface of the disk 3. Various magnetic "tracks" of information can be read from the magnetic disk 3 as the actuator 4 is caused to pivot in a short arc as indicate by the arrows P. The design and manufacture of magnetic disk drives is well known to those skilled in the art.
A magnetoresistive (MR) sensor is used to detect magnetic field signals by means of a changing resistance in a read element. A conventional MR sensor utilizes the anisotropic magnetoresistive (AMR) effect for such detection, where the read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. When there is relative motion between the MR sensor and a magnetic medium (such as a disk surface), a magnetic field from the medium causes a change in the direction of magnetization in the read element, thereby causing a corresponding change in resistance of the read element. The change in resistance can be detected to recover the recorded data on the magnetic medium.
MR sensors exhibit a number of desirable characteristics. For example, an MR sensor exhibits good linear density resolution and has an output signal strength that is independent of the relative velocity between sensor and medium.
In order for an MR sensor to operate properly, two bias fields are generally required, namely a longitudinal bias field and a transverse bias field. The longitudinal bias is used to suppress Barkhausen noise which is generated by multi-domain activities within the MR element and to improve the magnetic stability in the presence of high magnetic field excitation. The transverse bias field is used to bias the MR material so that its response to a magnetic field is in a linear range and of a high differential magnitude.
The transverse bias field is normal to the plane of the magnetic media and parallel to the surface of the planar MR element, and is usually provided by a layer of soft magnetic material deposited proximate to the MR element and magnetized by a magnetic field generated by a current flow in the MR element. The soft magnetic material is often referred to as a "Soft Adjacent Layer" or "SAL" and, as such, this form of biasing is often referred to as "SAL biasing." The SAL of the prior art is separated from the MR element by a thin nonmagnetic layer.
MR sensors using a SAL for transverse bias often exhibit magnetic instability in the sensor end or tail region and significant side-track reading. For high density storage, the height of the MR element is relatively small, e.g. less than 1 micrometer (um). For element dimensions in this range, it may not be possible to fully saturate the soft magnetic layer and therefore it may not provide an adequate transverse bias field to the MR sensor. For certain structural configurations, the unsaturated soft magnetic layer may also cause Barkhausen noise in the sensor. What is needed then, is a method for stabilizing the soft magnetic layer and insuring that it is saturated.
One method for achieving the above-stated goal is the use of the phenomenon of exchange anisotropy. It occurs as a result of the interaction of a ferromagnetic material in contact with an antiferromagnetic material, and can be described in terms of an exchange interaction between magnetic moments on each side of the interface between the two materials. For example, exchange coupling between thin layers of ferromagnetic nickel-iron (NiFe) and antiferromagnetic iron-manganese (FeMn) produces a unidirectional anisotropy. Improved transverse bias schemes for AMR sensors in which a SAL is stabilized by exchange coupling to an antiferromagnetic (AF) layer was proposed by Hardayal S. Gill et al. in U.S. Pat. No. 5,508,866.
Another form of magnetoresistance is referred to as giant magnetoresistance (GMR) which has been observed in a variety of magnetic multilayered structures. GMR sensors include at least two ferromagnetic metal layers separated by a non-ferromagnetic metal layer, and often include multiple such layers. For example, the GMR effect has been found in a variety of systems, such as Fe/Cr or Co/Cu multilayers. With GMR, the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers which, in turn, influences a change in the spin dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure therefore changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. A CIP GMR ML sensor provides a "current-in-plane" (CIP) sensing current as is well known to those skilled in the art.
In FIG. 1C, a shielded magnetoresistive head (MRH) 10 includes a first shield 12, a second shield 14, and a MR sensor 16 disposed within a gap (G) between shields 12 and 14. The MR sensor may be centered in the gap G, or may be offset to provide a self-bias, as is well known to those skilled in the art. Lines of magnetic flux impinging upon the MR sensor to create a detectable change in resistance. An air bearing surface S is defined by the MRH 10. For the purpose of this discussion, the MR sensor can be an AMR sensor or a GMR sensor. The design and manufacture of magnetoresistive heads, such as MRH 10, is well known to those skilled in the art.
In FIG. 2 a cross-sectional view taken along line 2--2 of FIG. 1C illustrates the structure of a MR sensor 16 of the prior art. The MR sensor 16 includes an MR sensing element 18, a spacer layer 20, a SAL layer 22, and antiferromagnetic (AFM) layer 24. Again, the MR element 18 can be either an AMR element or a GMR element. The SAL layer is typically made from a high resistivity, soft ferromagnetic material such as doped Permalloy. As is well known to those skilled in the art, Permalloy is a magnetic material nominally including 81% nickel (Ni) and 19% iron (Fe). The spacer layer 20 is typically a nonmagnetic metal such as tantalum for AMR, and copper for GMR. The AFM layer 24 is used to set the magnetic direction of the SAL 22, as will be discussed in greater detail below.
As is also seen in FIG. 2, the MR sensor 16 is supported by a substrate 17 and a buffer layer 19. Ferromagnetic end regions 21 abut the ends of the sensor 16. Leads 25, typically made from gold or other low resistance material, bring the current to the sensor 16. A capping layer 27 is provided over the AFM layer 24. A current source 29 provides a current I.sub.b to flow through the various layers of the sensor 16, and signal detection circuitry 31 detects changes in resistance of the sensor 16 as it encounters magnetic fields.
The purpose of the SAL 22 will be discussed with reference to FIGS. 3 and 4. In FIG. 3, the MR element 18 can have a total magnetization as illustrated by the arrow 26, while the SAL 22 is magnetized as indicated by the arrow 28. Absent the magnetic coupling of the SAL, the MR sensor may have a magnetization as indicated by the dashed arrow 30. The actual magnetic angle 26 is the sum of the magnetic angle 30 and the magnetostatically coupled magnetic field 28 of the SAL.
As seen in FIG. 4, the magnetization 28 of the SAL 22 at a right angle to the magnetization 30 of the MR element 18 biases the free element to a point 32 (with a 45.degree. angle) on a R vrs. H curve 34 that is relatively linear as indicated by the dashed line 36, and which has a relatively large slope m. Linearity is, of course, desirable to provide a linear response, and the relatively large slope is desirable in that it produces large resistance changes in response to the changes in the magnetic field.
The SAL biasing method has a shortcoming in that a relatively high current (e.g. &gt;10.sup.7 amp/cm.sup.2) is needed to saturate the SAL to obtain an adequate biasing level. This becomes very pronounced as the height of the AMR element is reduced to 1 micron or less. Additionally, as noted previously, AMR sensors using a SAL transverse bias often exhibit magnetic instability in the sensor or tail region and significant side-track reading. Further, for GMR ML, GMR magnitude was observed to decrease with increasing sense current due to a rise in the temperature of the sensor elements and, therefore, GMR must be optimized for a realistic sense current.